Containers of the aforementioned type serve as inner containers of so-called dual-material fluid receptacles having an outer cover and can be made of a soft thermoplastic material, such as polyethylene; it is then required that a molded and threaded fluid-tight connecting piece, fabricated of a relatively thick-walled and stable material, can be inserted into the opening of the flexible container and coupled with the latter's thin wall firmly and in a fluid-tight fashion so that transportation or other manipulation of that assembly do not destroy that connection or cause it to leak.
It is known to form a flexible container out of a plasticized foil made of synthetic resin by a blowing or vacuum process and to subsequently attach a relatively thick-walled and stable pre-fabricated connecting piece into that opening by means of clamps. The connecting piece is then provided on its base with an attachment flange which, upon insertion into the opening of that container, grips the rim of that opening from the interiors, a clamping ring exerting counter-pressure from the exterior and snapping over a bulge of the connecting piece pushing the rim of the container into a circular recess of the attachment flange--see German Patent 1,249,718. In order to obtain an adequate degree of sealing at the point of attachment such purely mechanically functioning attachment pieces were required to fit into each other very firmly, which in turn led to frequent breakage and damage of these pieces during the attachment process. Furthermore, such packaging required considerable preparation and manual labor and did not guarantee sealing effectiveness or durability.
To obviate such disadvantages many experiments have been undertaken to weld the thin container wall to the attachment flange of the connecting piece either by heat treatment or ultrasonically. Numerous difficulties were encountered, partly due to the unfavorable properties of the soft plastic, and partly due to the impossibility of obtaining a sufficiently isolating counterpressure from the inside of the container. The closed bottom of the container did not permit any access from its underside, so that the connecting piece made contact with the bottom during the welding process and was welded thereto by a so-called remote welding effect. The resultant considerable thickening of the attachment flange turned out to be disadvantageous because the material of the thin-walled container was considerably changed in structure in the bottom region of the container as well as in the supporting region of the connecting piece during welding. This lack of homogeneity of the container material within the attachment region and the container bottom led to stress and failure during transportation or due to falls, or as a result of corrosive liquids used for filling the containers. Thus methods used to date for welding synthetic resin appeared inapplicable. It also was not possible to make use only of the general teachings of manufacturers of ultrasonic welding devices, which provide the parts to be welded with an energy-focusing device projecting from one of the connecting surfaces since the problem of welding thin-walled or foil-like closed containers to thick-walled containers or closure parts had not been dealt with in any of these proposals.